The present invention relates generally to a connector and, more specifically, to an optical coupler for single fiber optic cables.
The employment of fiber optic cables or light guides, also sometimes referred to as optical communication fibers, for the tansmission of information-bearing light signals, is now an established art. Much development work has been devoted to the provision of practical low-loss glass materials and production techniques for producing glass fiber cables with protective outer claddings or jackets. The jackets make them resemble ordinary metallic-core electrical cable upon superficial external inspection. Obviously, if fiber optic cables are to be used in practical signal transmission and processing systems, practical connectors for the connection and disconnection of fiber optic cables must be provided.
Some references will now be given for background in the state of fiber optic art in general. An article entitled, "Fiber Optics," by Narinder S. Kapany, published in Scientific American, Vol. 203, pgs. 72- 81, November 1960, provides a useful background in respect to some theoretical and practical aspects of fiber optic transmission.
Of considerable relevance to the problem of developing practical fiber optic connectors, is the question of transfer efficiency at the connector. Various factors, including separation at the point of abutment, and lateral separation or axial misalignment, are among the factors effecting the light transfer efficiency at a connector. In this connection, attention is directed to the Bell System Technical Journal, Vol. 50, No. 10, December 1971, specifically to an article by D. L. Bisbee, entitled, "Measurement of Loss Due to Offset, and End Separations of Optical Fibers." Another Bell System Technical Journal article of interest appeared in Vol. 52, No. 8, October 1973, and was entitled, "Effect of Misalignments on Coupling Efficiency on Single-Mode Optical Fiber But Joints," by J. S. Cook, W. L. Mammel and R. J. Grow.
Fiber optic bundles are normally utilized for only short transmission distances in fiber optic communications networks. On the other hand, fibers are used individually as optical data channels to allow transmission over many kilometers. At present, most fiber optic cables are multi-fiber bundles due to less stringent splicing requirements greater inherent redundancy and higher signal-to-noise ratio. The difficulty in achieving connections between single fibers which are insensitive to axial misalignment problems has created an obstacle to the use of long run single data transmission sytems.
Therefore, a connector or coupler is required to eliminate lateral tolerances if low-loss connections are to be obtained in the use of single fiber optical transmission arrangements. "V" groove and metal sleeve arrangements have been used to interconnect single fibers. Reference is made to U.S. Pat. No. 3,768,146 which discloses a metal sleeve interconnection for single fibers. The problem in achieving alignment between single fibers is enhanced due to the typical lack of concentricity between the fiber core and its outside cladding or jacket. Thus, even if the optical fiber cables are perfectly aligned, the cores therein may be laterally displaced. Also, in typical single fiber coupling arrangements the end faces of the transmitting and receiving fibers abut one another, which may cause scratches on the end faces and thus light transmission losses. Further, if one fiber is slidably inserted into the end of an alignment tube fixedly retaining the other fiber, as is required for a separable connector, one or both of the fibers may fracture if axially compressed during the interconnection. If the ends of the transmitting or receiving fibers were simply inserted into the ends of an alignment tube, in axially spaced relationship, some light transmission would be lost due to the fact that the receiving fiber would not completely fill the optical beam passing from the transmitting fiber through the tube. If a shoulder were formed in the bore in the tube against which the receiving fiber could be positioned, light transmission losses between the outer surface of the fiber and the wall of the bore in the tube would be eliminated. However, it is virtually impossible to maintain manufacturing tolerances such as to prevent the shoulder from extending over the peripheral region of the core of the fiber, with the result that additional light transmission losses would be suffered. Therefore, what is needed and constitutes the purpose of the present invention is to provide a coupling arrangement for a pair of single optical fibers which may be incorporated into a connector for practical field utilization, which is insensitive to fiber core alignment problems and problems in lack of concentricity between the core and the cladding of the fiber cable, and in which the receiving fiber receives the entire optical beam transmitted from the transmitting fiber.